The present invention relates to water- and ion-conducting membranes used for fuel cells, for heat and moisture exchange in heating/ventilation/air conditioning systems and for desalination.
Ion conducting membranes of various compositions are known. An overview of the subject is provided in Vincent, C. A., Polymer Electrolyte Reviews I (1987). Many ion-conducting polymers also conduct water. Ion conducting polymers composed of sulfonated hydrogenated block copolymers of styrene and butadiene are disclosed by Ehrenberg et al. in U.S. Pat. Nos. 5,468,574 and 5,679,482, the entire disclosure of which is incorporated herein by reference. The copolymers are described as useful for membranes in fuel cells. No other uses of the copolymers are mentioned.
A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. Since the process does not xe2x80x9cburnxe2x80x9d the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes separated by an ion-conducting membrane. The fuel gas (e.g., hydrogen) is ionized on one electrode, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the other electrode. If current is not allowed to run from one electrode to the other, a potential gradient is built up to stop the diffusion of the hydrogen ions. Allowing some current to flow from one electrode to the other through an external load produces power.
The membrane separating the electrodes must allow the diffusion of ions from one electrode to the other, but must keep the fuel and oxidant gases apart. It must also prevent the flow of electrons. Diffusion or leakage of the fuel or oxidant gases across the membrane can lead to explosions and other undesirable consequences. If electrons can travel through the membrane, the device is fully or partially shorted out, and the useful power produced is eliminated or reduced.
It is therefore an object of this invention to produce a membrane which allows the diffusion of ions, specifically protons, but prevents both the flow of electrons and the diffusion of molecular gases. The membrane must also be mechanically stable and free of porosity and pinholes which would allow passage of molecular gases.
In constructing a fuel cell, it is particularly advantageous that the catalytic electrodes be in intimate contact with the membrane material. This reduces the xe2x80x9ccontact resistancexe2x80x9d that arises when the ions move from the catalytic electrode to the membrane and vice versa. Intimate contact can be facilitated by incorporating a material having the same composition as the membrane into the catalytic electrodes as a binder. [See Wilson and Gottesfeld J. Appl. Electrochem. 22, 1-7 (1992)] It is therefore an object of the invention to produce a membrane wherein such intimate contact is easily and inexpensively made.
For reasons of chemical stability, fuel cells presently available typically use a fully fluorinated polymer such as Dupont""s Nafion(copyright) as the ion-conducting membrane. This polymer is expensive to produce, which raises the cost of fuel cells to a level that renders them commercially unattractive. It is therefore a further object of this invention to produce an inexpensive ion-conducting membrane.
Membranes composed of hydrophilic polymers have been used in heating, ventilating and air conditioning systems to improve control of humidity while reducing energy costs. Systems function by allowing transfer of moisture between a humid air stream to a relatively dry one. One of the functions of a HVAC (heating/ventilation/air conditioning) system in a building is to exhaust air to the atmosphere and simultaneously replenish the exhausted air with fresh air. It is necessary to adjust the temperature of the fresh air to approximately the same temperature and humidity of the exhausted air before introducing it into the building. This requires additional cooling or warming of the fresh air and the addition or removal of moisture, at a significant energy cost. In addition, this ventilating process frequently employs moving parts in the apparatus which requires periodic maintenance. In order to minimize energy and maintenance costs, it is desirable to provide a static heat and moisture exchanging core for simultaneously and continuously effecting both heat and moisture exchange between two air streams. An inexpensive water-conducting membrane having mechanical strength is desirable in order to provide an improved operating lifetime for such cores. U.S. Pat. No. 4,051,898 to Yoshino discloses the use of Japanese paper to transfer heat and moisture between fresh intake air and exhaused room air in an HVAC system. Zhang and Jiang (J. Membrane Sci., pages 29-38 (1999)) disclose an energy recovery ventilator wherein heat and water are transferred across a porous hydrophilic polymer membrane. In U.S. Pat. No. 5,348,691, McElroy et al. disclose a humidifying device wherein water is transported across a membrane composed of a perfluorocarbonsulfonic acid polymer or a polystyrenesulfonic acid. It is therefore an object of this invention to produce a membrane which allows the transfer of water between two gas streams separated by the membrane. The membrane must also be mechanically stable and free of porosity and pinholes which would allow clogging by contaminants.
Existing desalination plants are typically based on reverse osmosis membranes. These membranes are designed such that water can pass through, leaving behind salts and minerals. Due to water concentration differences between the two surfaces of the membrane, a physical assist in the form of a pressure differential is required for water to pass through the membrane. Therefore, the seawater is pressurized in order to force water through the membrane. One undesirable effect of applying pressure to the seawater is that contaminants that are too large to pass through the membrane are forced against it, reducing the efficiency of the membrane. Therefore, the membrane must be periodically backflushed or surface scoured to remove the contaminants. In order to guarantee that the reverse osmosis plant can sustain the rated potable water production, the plant must be oversized to allow for concurrent membrane flushing while still producing clean water.
The reverse osmosis process draws a considerable amount of energy to pump seawater through the membrane. The physical plant is costly due to the complexity of the piping necessary to support the pressurized operation with the necessary membrane cleaning. In addition, disposal of effluent from the plant requires that contaminants, which are concentrated by the reverse osmosis process, must be rediluted to be safely disposed of.
Therefore, there is a need for a cost-effective alternative to the reverse osmosis process for the production of potable water from brine.
In one aspect, the present invention relates to a water- and proton-conducting membrane according to the present invention comprises a sulfonated statistical copolymer. This statistical copolymer comprises an arylvinyl monomer and at least one monoolefin monomer, and aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated. The statistical copolymer preferrably comprises from about 20 weight percent to about 80 weight percent arylvinyl monomer. The arylvinyl monomer is preferably styrene, vinyl toluene, or xcex1-methylstyrene, and, more preferably, is styrene.
Aromatic moieties derived from the arylvinyl monomer preferably comprise from about 20 mole percent aromatic sulfonate to about 80 mole percent aromatic sulfonate, more preferably, from about 20 mole percent aromatic sulfonate to about 50 mole percent aromatic sulfonate, and, most preferably, from about 30 mole percent aromatic sulfonate to about 50 mole percent aromatic sulfonate.
In a preferred embodiment, the statistical copolymer comprises from about 20 weight percent to about 80 weight percent styrene. Aromatic moieties derived from styrene preferably comprise from about 20 mole percent styrene sulfonate to about 80 mole percent styrene sulfonate, more preferably, from about 20 mole percent styrene sulfonate to about 50 mole percent styrene sulfonate, and most preferably, about 30 mole percent styrene sulfonate to about 50 mole percent styrene sulfonate.
The monoolefin monomer is preferably ethylene, propylene, 1-butene, 2-butene, isobutylene, 1-pentene, 4-methyl-1-pentene, 1-hexene, or 1-octene. The statistical copolymer may additionally comprise norbornene. A preferred monoolefin monomer is ethylene. More preferably, the statisitical copolymer comprises a copolymer of styrlene and ethylene. The weight-average molecular weight (Mw) of the statistical copolymer is at least 20,000.
In another aspect, the present invention relates to a fuel cell comprising the water- and proton-conducting membrane described above, first and second opposed electrodes in contact with said proton conducting membrane, means for supplying a fuel to said first electrode; and means for permitting an oxidant to contact said second electrode. Preferably, at least one of said first and second electrodes comprises catalytic particles and a sulfonated statistical copolymer of styrene and ethylene as a binder. More preferably, the sulfonated statistical styrene copolymer is a sulfonated reduced statistical styrene butadiene copolymer or a sulfonated statistical styrene ethylene copolymer. The present invention also relates to a fuel cell comprising at least one humidification section for humidification of a fuel gas, said humidification section comprising a membrane having at least two opposed surfaces disposed between a fuel stream and a water-containing fluid, said membrane comprising a sulfonated statistical styrene copolymer.
In yet another aspect, the present invention relates to a method of conditioning air for an enclosure by transferring heat and moisture between a first stream of outside ambient air and a second stream of enclosure return air comprising disposing a water-conducting membrane between said first and second stream, said water-conducting membrane having at least two opposed surfaces and comprising a sulfonated statistical copolymer, the statistical copolymer comprising at least one arylvinyl monomer and at least one olefin monomer, and wherein aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated; and contacting the first and second gas stream with an opposite surface of said water-conducting membrane, whereby heat and moisture are transferred from the first stream of outside ambient air to the second stream of enclosure return air. The sulfonated statistical copolymer is preferably a sulfonated reduced statistical styrene butadiene copolymer or a sulfonated statistical styrene ethylene copolymer. A heat and moisture exchanger core for transferring heat and moisture between a first stream of outside ambient air and a second stream of enclosure return air comprises a water-conducting membrane disposed between the first stream of outside ambient air and the second stream of enclosure return air, said water-conducting membrane comprising a sulfonated statistical styrene copolymer; whereby heat and moisture are transferred from the first stream of outside ambient air to the second stream of enclosure return air. An apparatus for conditioning air for an enclosure comprising a heat and moisture exchanger core for transferring heat and moisture between a first stream of outside ambient air and a second stream of enclosure return air, said heat and moisture exchanger core comprising a water-conducting membrane disposed between a first stream of outside ambient air and a second stream of enclosure return air, said water-conducting membrane comprising a sulfonated statistical copolymer, the statistical copolymer comprising at least one arylvinyl monomer and at least one olefin monomer, and wherein aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated; whereby heat and moisture are transferred from the first stream of outside ambient air to the second stream of enclosure return air.
In yet another aspect, the present invention relates to a process for the desalination of brine which does not have high energy requirements, does not require pressurized operation, or periodic flushing of a membrane and does not produce a concentrated effluent for disposal. A process for extracting potable water from a brine comprises providing a membrane comprising a sulfonated arylvinyl polymer, the arylvinyl polymer comprising at least one arylvinyl monomer and wherein aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated; placing a first surface of the membrane in contact with the brine; passing a gas over a second surface of the membrane, the gas being initially at least partially unsaturated with water vapor; and cooling the gas to condense liquid potable water. Preferably, the sulfonated styrene copolymer is a sulfonated styrene-ethylene-butylene-styrene triblock copolymer, a sulfonated reduced statistical styrene butadiene copolymer or a sulfonated statistical styrene ethylene copolymer. The process may additionally comprise heating the gas before passing the gas over the second surface of the membrane. The steps are preferably performed repeatedly in sequence. The gas may be heated or cooled by seawater. An apparatus for the extraction of potable water from a brine comprises a membrane comprising a sulfonated arylvinyl polymer, the arylvinyl polymer comprising at least one arylvinyl monomer and wherein aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated, a first surface of said membrane contactable with the brine, and a second surface of said membrane contactable with a stream of gas, the gas being at least partially unsaturated with water vapor; a cooling surface contactable with the gas; and a warming surface contactable with the gas.